Halftoning may be described as applying a high frequency/low frequency dichotomy having a low frequency attribute and a high frequency attribute. For example, the low frequency attribute may be a local area of the output image designated as a halftone cell. Each equal-sized cell relates in size and location to a corresponding area of the continuous-tone input image. Within each cell, the high frequency attribute may be a centered, variable-sized, halftone dot or pixel composed of a colorant such as ink or toner.
Single level digital halftoning uses a raster image or bitmap within which each monochrome picture element, or pixel, may be on or off, that is, tone or no tone, ink or no ink. The ratio of the inked area to the non-inked area of the output cell corresponds to the luminance or gray level of the input cell. From a suitable distance, the human image processing averages both the high frequency apparent gray level approximated by the ratio within the cell and the low frequency apparent changes in gray level between adjacent equally-spaced cells and centered dots. To achieve this effect, the digital halftone cell contains groups of monochrome pixels within the same-sized cell area. Printing devices that are capable of printing objects rendered in pixels having two or more density levels, that is two or more gray levels, have a second means of approaching the appearance of a continuous tone scale over the binary state afforded single level halftoning. Accordingly, multilevel halftoning the application of two or more density levels for toning or pigmenting may be applied in conjunction with halftoning to produce refined renderings.